1. Field of the Invention
The present invention relates to a display device and method for operating the same. Specifically, the invention relates to a display device using an active matrix type liquid crystal panel. Note that the present invention is also applicable to a display device having a display panel that uses as a display medium other material than liquid crystal.
2. Description of the Related Art
Rapid development has been made in recent years in a technique for manufacturing a semiconductor device, for example, a thin film transistor (TFT), which has a semiconductor thin film formed on an inexpensive glass substrate. This is because there is an increasing demand for active matrix type liquid crystal display devices (liquid crystal panels).
In an active matrix type liquid crystal display device, a TFT is disposed in each of several hundred thousands to several million pixels which are arranged in matrix in a pixel region, and electric charge flowing in and out of each pixel electrode is controlled by switching function owned by the TFTs.
In the pixel region, thin film transistors using amorphous silicon formed on a glass substrate are arranged.
A structure is known in which quartz is utilized as a substrate and thin film transistors are fabricated from a polycrystalline silicon film. In this case, the thin film transistors formed on the quartz substrate are used to form both of a peripheral driver circuit and a pixel portion.
Also known is a technique in which thin film transistors using a crystalline silicon film are formed on a glass substrate by laser annealing or other technologies. Employment of this technique allows of integrating a pixel portion and a peripheral driver circuit on the glass substrate.
The active matrix type liquid crystal panels have lately been adopted in many notebook type personal computers. In personal computers, multi-gray scale liquid crystal panels are needed in order to, e.g., simultaneously start a plurality of software, or process images taken in from a digital camera.
The demand for liquid crystal projectors for a large screen that is capable of displaying thereon images by High-vision (a broadcast standard developed by NHK) signals has been growing. It is true also for such projectors that the quality of displayed images depends on how fine the gray scale display is.
As just has been mentioned, a key factor for providing a high quality image is how finely the gray scale display can be set. There are two types of gray scale display, one is to feed a source line with an analog signal such as a video signal or a television signal (analog gray scale), and the other is to feed the source line with a digital signal such as a data signal output by a personal computer (digital gray scale).
In analog gray scale, as described above, analog image signals to be fed to image signal lines are sequentially selected in response to a signal from a source driver, and predetermined image signals are fed to corresponding source lines.
In digital gray scale, digital signals to be fed to the image signal lines are sequentially selected and subjected to D/A conversion, and then predetermined image signals are fed to corresponding source lines.
A relation as expressed by the dotted line in FIG. 17 is established between voltage (V) applied to liquid crystal and the intensity of transmitting light in liquid crystal panels irrespective of the type of gray scale display, provided that the liquid crystal panels use normally white mode in which the panels are bright when voltage is not applied in TN (twist nematic) mode.
As can tell from FIG. 17, a nonlinear relation is found between the voltage applied to the liquid crystal and the intensity of transmitting light which is the cause of difficulty in making the fineness of gray scale display respond to the magnitude of the applied voltage. Therefore, images cannot be accurately reproduced from image signals input from an external, and to provide good quality images is difficult.